PSI - Issue 24

Giuseppe Mirone et al. / Procedia Structural Integrity 24 (2019) 259–266 Mirone & Barbagallo / Structural Integrity Procedia 00 (2019) 000 – 000

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which tends to delay the necking. 3.3. Combined strain rate and temperature effect on necking in dynamic tests

In this section, the main aim is to analyze the necking onset during dynamic tests in order to evaluate the influence on it of the strain rate and of the adiabatic temperature generated during the tests. In the left part of Fig. 3, the dynamic true stress-true strain curves are compared to the static one at room temperature. The dynamic curves obviously include the thermal softening due to the adiabatic heating; in fact, despite at low strains the dynamic curves are higher than the static reference one, at later stages the effect of the temperature becomes more and more relevant and at high strains the dynamic ones are even lower than the static reference ones. A clear strain-rate effect is then visible, with the dynamic curves ordered for increasing rates, so the curves from 15 kN incident wave tests are lower than those from the tests with 26 kN incident waves. In the right part of Fig. 3, the true strain rate vs. true strain evolution according to eq. (7) is shown for each dynamic test.

Fig. 3. Experimental True Stress-True strain Dynamic Curves (left) and True strain rate vs. true strain Curves (right) from Dynamic tests The dynamic true curves are then back-corrected by means of the post-necking function in order to obtain the corresponding equivalent curves. Their underlying area, via the Taylor-Quinney coefficient (TQC), allows to calculate the adiabatic heating produced during each dynamic test. In this work the TQC has been considered equal to one, according to Kapoor & Nemat-Nasser (1998). The evolving temperature is then obtained during each test, as it is shown in Fig. 4, and the softening effect is finally removed from the experimental curves, by way of such temperature histories introduced into the softening function of Fig. 2. The obtained true stress – true strain curves combined to the temperature histories have been analyzed in order to evaluate the necking inception strain and the relative temperature at that specific instant. The results of such analysis is shown in Table 3. The necking strains from static and dynamic test are plotted in Fig. 5.

Table 3. Necking strain and temperature at necking for dynamic tests D-01 (15 kN) D-02 (15 kN) D-03 (26 kN) D-04 (26 kN) ε N−D ͲǤʹͶ ͲǤʹͶ ͲǤͳͻ ͲǤʹ͵ T N ͹ͷ ͹͸ ͸ͷ ͹ͷ